Download Colour - Magnitude Diagram for M 45

Colour - Magnitude Diagram for M 45
(Pleiades)
Introduction
The Pleiades is a relatively close open cluster. The six or seven stars visible to the
naked eye form a tight grouping of stars (an asterism) near the even closer Hyades
cluster. They are easily visible in the winter sky in the northern hemisphere. In this
exercise, you will determine the colour of many cluster members and plot them on a
Colour-Magnitude diagram. This is just a type of Hertzsprung-Russell (HR) diagram
in which we plot Colour Index rather than Spectral Class on the horizontal axis; and
use the apparent visual magnitude, V, for the vertical axis.
Procedure
Photometric measurements of the Pleiades cluster can be used to determine the age of
the cluster and its distance. By taking images of the stars through separate blue (B)
and visual (V) filters we can measure the apparent magnitude of each star in each
waveband colour. In this exercise this stage has already been done, presenting you
with a table with two magnitude values for each star. A visual or V filter approximates
the spectral response of the human eye and is most sensitive in the yellow part of the
spectrum. A blue filter, B, corresponds to the sensitivity of photographic film (hence
photographic magnitude).
The Data is on an excel spreadsheet accompanying this information
1. Calculate and record the Colour Index for each star. The colour index or CI is
found by the following equation:
CI = B - V
2. Plot your results as an X- Y scatter plot, using V for the vertical axis and B - V
for the horizontal axis. Remember to draw your vertical scale so that the lower
the value of V, the higher up the axis it is. Ensure that you have clearly
marked the scales. Give your self enough room above and below your vertical
scale values to add additional data later.
Questions
1.
2.
3.
4.
5.
What type are the majority of stars in the Pleiades cluster?
In blue, circle the most massive star/s on your Colour-Magnitude plot.
In red, circle the least massive group of stars on the diagram.
What is the source of fuel for all the stars shown on the diagram?
Comment on the relative age of the stars. Are they young or old? How can you
tell?
6. Look at the photo of the Pleiades below. Is there any visible evidence to
support your answer to question 5? What does this evidence suggest about the
origin of stars?
The Pleiades Open Cluster, M45.
7. Where in the galaxy would you typically expect to find open clusters like the
Pleiades?
8. In this exercise, you have plotted apparent magnitude V or mv, rather than
absolute magnitude, Mv, or Luminosity on the vertical axis. What assumption
about the stars in the Pleiades cluster must be made for this to be valid?
9. Results from the European astrometric satellite, HIPPARCOS, gave a distance
of 116 parsecs to the Pleiades. Using the distance modulus equation:
m - M = 5log10(d/10)
Calculate the Absolute (visual) Magnitude, M of each of the Pleiades stars
(easy if your data is already on a spreadsheet). You will have to rewrite the
equation first.
10. On the right hand vertical axis of your plot, recalibrate the scale using
Absolute Magnitude rather than V.
11. The value m - M is called the distance modulus. What do you notice about this
value for all the Pleiades stars? What is the value for M 45?
12. Would the value of the distance modulus for a more distant cluster be higher
or lower? Explain.
13. The relationship between Colour Index and Spectral Class for Main Sequence
stars (ie those of Luminosity Class V) is shown in the table below. Use it to
mark in the values for spectral class beneath those of colour index on your
plot.
Luminosity Classes of Stars
Spectral Class B - V
O5
-0.35
B0
-0.31
B5
-0.16
A0
0.00
A5
+0.13
F0
+0.27
F5
+0.42
G0
+0.58
G5
+0.70
K0
+0.89
K5
+1.18
M0
+1.45
M5
+1.63
14. On your plot, write in the colour (eg, red) beneath the corresponding spectral
class.
15. Using a capital S, mark in where the Sun would be on your plot (use M).
16. Draw and label three regions on your plot to show where red giants, red
supergiants and white dwarfs would be found.
17. Below is a colour-magnitude diagram for the Hyades, the nearest open cluster
to Earth. Compare this with the one you have plotted for the Pleiades. Which
of the two open clusters, the Pleiades or the Hyades is older? How can you
tell?
Below is a colour-magnitude diagram for the globular cluster, M5. Is the globular
cluster older or younger than the open clusters? Justify your answer
© Copyright CSIRO Australia 2004